An electric power steering apparatus which provides a steering mechanism of a vehicle with a steering assist torque by means of a rotational torque of a motor, applies a driving force of the motor as the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus (EPS) performs feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of duty command values of pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a steering shaft (a column shaft or a handle shaft) 2 connected to a steering wheel 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack-and-pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the steering shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1, and a motor 20 for assisting a steering force of the steering wheel 1 is connected to the steering shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist (steering assist) command on the basis of a steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 on the basis of a voltage control value Vref obtained by performing compensation or the like with respect to the current command value. Moreover, it is also possible to receive the vehicle speed Vel from a controller area network (CAN) or the like.
The control unit 30 mainly comprises a CPU (also including an MCU, an MPU or the like), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Th detected by the torque sensor 10 and the vehicle speed Vel detected by the vehicle speed sensor 12 are inputted into a current command value calculating section 31 for calculating a current command value Iref1. The current command value calculating section 31 calculates the current command value Iref1 that is a control target value of a current supplied to the motor 20 on the basis of the inputted steering torque Th and the inputted vehicle speed Vel and by using an assist map or the like. The current command value Iref1 is inputted into a current limiting section 33 through an adding section 32A. A current command value Irefm the maximum current of which is limited is inputted into a subtracting section 32B, and a deviation I (=Irefm−Im) between the current command value Irefm and a motor current value Im being fed back is calculated. The deviation I is inputted into a PI control section 35 for characteristic improvement of steering operations. The voltage control value Vref whose characteristic is improved by the PI control section 35 is inputted into a PWM control section 36. Furthermore, the motor 20 is PWM-driven through an inverter circuit 37 serving as a driving section. The current value Im of the motor 20 is detected by a motor current detector 38 and is fed back to the subtracting section 32B. The inverter circuit 37 uses FETs as driving elements and is comprised of a bridge circuit of FETs.
Further, a compensation signal CM from a compensating section 34 is added in the adding section 32A, and compensation of the system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic or the like. The compensating section 34 adds a self-aligning torque (SAT) 343 and an inertia 342 in an adding section 344, further, adds the result of addition performed in the adding section 344 and a convergence 341 in an adding section 345, and then outputs the result of addition performed in the adding section 345 as the compensation signal CM.
Such an electric power steering apparatus has been conventionally equipped with a special steering angle sensor in order to detect a steering wheel angle. Recently, however, there is the case where it is equipped with an angle sensor that can be also made function as a torque sensor by requests such as improvement of reliability, redundancy of functions, and cost reduction.
In general, a steering wheel of a vehicle is designed to rotate from a neutral point to the left and the right by about one and half rounds. In other words, the steering wheel can rotate from a left end to a right end by about three rounds. Therefore, an electric power steering apparatus must be equipped with a steering angle sensor compatible with multi-turn that can detect a wide range of the angle being more than or equal to three rounds in order to appropriately detect the steering wheel angle. Those rounds are equivalent to more than or equal to 1080 degrees (360 degrees×3) as converted into the angle, and it is desirable that the sensor can detect the angle about 1400 degrees in consideration of a margin or the like. In order to respond to this request, an angle sensor incorporating a reduction mechanism or the like has been conventionally constructed, and has been used as a steering angle sensor. However, providing the sensor with the reduction mechanism or the like makes the structure complicated and the cost increase, so that a sensor has been required that makes installation of the steering angle sensor omitted and replaces the steering angle sensor.
On the other hand, an electric power steering apparatus using a brushless DC motor provides a motor shaft with a resolver as a high-precision rotation angle sensor in order to secure commutation accuracy of a motor. However, the resolver detects an angle with high accuracy, while a detectable range of the angle is limited, generally one period of an electric angle, so that the resolver is not suitable to detect a wide range of the angle. Therefore, for example, in the case that a scale factor of reduction performed by a reduction mechanism arranged between a motor and a steering shaft performs is “18.5”, and a scale factor depending on the number of pole pairs in the motor is “3”, a change in an angel of the steering shaft is detected as a change in an angle of the resolver with a scale factor of 55.5 times in total. That is, in the steering system where the steering wheel rotates from the left end to the right end by three rounds, the resolver angle repeatedly changes in 166.5 periods. This makes it difficult to estimate a steering wheel angle only by the resolver angle, so that there are problems such as that it is necessary to take longer time than a certain degree to perform some estimation. Things like estimation of a neutral point using a steering torque, a wheel speed or the like, estimation from one end to the other end, and estimation of a steering angle by a SAT.
Further, the recent requests such as improvement of reliability, redundancy of functions, and cost reduction have raised the case of equipping the steering shaft with an angle sensor that can also function as a torque sensor. In this case, the angle sensor can detect the angle of the steering shaft in ranges such as a period of 40 degrees and a period of 20 degrees in order to obtain resolution required as a torque sensor. However, even a capacity to detect the angle in the period of 40 degrees does not enable the steering system where the steering wheel rotates from the left end to the right end by three rounds to estimate the steering wheel angle without processing equivalent to in the case of the resolver angle because the angle repeatedly changes in 27 periods.